Minimal structural view of a MatExpansionPanel the set-sync needs:
a read/write expanded boolean and an expandedChange stream. Kept
host-agnostic (no @angular/material type in the signature) so the
helper is unit-testable with plain fakes, mirroring the Material-
agnostic shape of createMaterialBidirectionalSync in
@cngx/common/data. MatExpansionPanel satisfies it structurally
(expanded getter/setter from CdkAccordionItem, expandedChange
is an EventEmitter<boolean> whose .subscribe matches).